text: Hexose transporters (HXT) are a group of proteins that are largely responsible for the uptake of glucose in yeast.  In S. cerevisiae, 20 HXT genes have been identified and 17 encode for glucose transporters (HXT1-HXT17), GAL2 encodes for a galactose transporter, and SNF3 and RGT2 encode for glucose sensors.  The number of glucose sensor genes have remained mostly consistent through the budding yeast lineage, however glucose sensors are absent from Schizosaccharomyces pombe.  Sch. pombe is a Crabtree-positive yeast, which developed aerobic fermentation independently from Saccharomyces lineage, and detects glucose via the cAMP-signaling pathway.  The number of transporter genes vary significantly between yeast species and has continually increased during the evolution of the S. cerevisiae lineage.  Most of the transporter genes have been generated by tandem duplication, rather than from the WGD.  Sch. pombe also has a high number of transporter genes compared to its close relatives.  Glucose uptake is believed to be a major rate-limiting step in glycolysis and replacing S. cerevisiae<nowiki/>s HXT1-17 genes with a single chimera HXT' gene results in decreased ethanol production or fully respiratory metabolism.  Thus, having an efficient glucose uptake system appears to be essential to ability of aerobic fermentation.  There is a significant positive correlation between the number of hexose transporter genes and the efficiency of ethanol production.